1. Field of the Invention
The present invention generally relates to electrophotographic printing machines, and more particularly to a serial-type electrophotographic printing machine equipped with an image fixing device provided in a carriage performing printing.
Recently, electrophotographic printing machines having a carriage operating in an electrophotographic process have been developed due to demands for cost reduction and down-sizing of electrophotographic recording machines. Generally, such electrophotographic printing machines operate so that a carriage is moved on a transfer device in the direction traverse (perpendicular) to the direction in which paper is transported, and an image transferred on the paper is fixed by means of a roller-shaped fixing device disposed in the paper transporting direction. In order to facilitate down-sizing, an electrophotographic printing machine has been developed in which the fixing device is provided in the carriage. Nowadays, it has been required that such an electrophotographic printing machine operates safely and the size thereof is further reduced.
2. Description of the Prior Art
FIGS. 1A and 1B show the structure of a carriage provided in a conventional electrophotographic printing machine. More particularly, FIG. 1A is a plan view of the carriage and its peripheral parts, and FIG. 1B is a cross-sectional view taken along a line I.sub.B --I.sub.B shown in FIG. 1A.
Referring to FIGS. 1A and 1B, a carriage 11 is made up of a process unit 12 and a fixing device 13, and is moved, by means of a driving motor (not shown for the sake of simplicity) above a transfer unit (print platen) 15 along guides 14a and 14b in the direction traverse to the paper transporting direction. On the either side of the carriage 11, transporting rollers 16a and 16b are arranged. A recording paper 17 is transported between the transfer unit 15 and the carriage 11 by means of the transporting rollers 16a and 16b.
The process unit 12 of the carriage 11 is equipped with an image carrier 21, which is rotated at a circumferential speed tuned with the movement of the carriage 11. The surface of the image carrier 21 is uniformly electrified by a charger 22. An electrostatic latent image is formed on the surface of the image carrier 21 by an exposure unit 23. The electrostatic image is visualized as a toner image by means of a developing roller 24. The toner image is transferred onto the recording paper 17 by means of the transfer unit 15 located at the side of the recording paper 17 opposite to the image carrier 21. The toner particles remaining on the image carrier 21 are scraped away by a cleaner 25.
The surface of the image carrier 21 which has been cleaned in the above way is electrified again by the charger 22, and the same process as described above is repeatedly performed. When the printing along a predetermined width is completed, the recording paper 17 is transported by a predetermined amount of movement by means of the transporting rollers 16a and 16b. The carriage 11 is returned to the predetermined position (home position), and performs the printing again. The home position is defined as a position which is located above the transfer unit 15 but is not located above the recording paper 17.
The image transferring by the transfer unit 15 is performed by applying a voltage across the image carrier 21 and the transfer unit 15, an electrically conductive member 15b such as an electrically conductive rubber sheet is placed on a base 15a.
The fixing unit 13 is equipped with a fixing roller 26 in which a heat source 27 such as a halogen lamp is provided. The fixing roller 26 is pressed against the recording paper 17. A silicon oil coating unit 28 is provided so as to be in contact with the fixing roller 26 in order to facilitate detachment of the toner particles from the fixing roller 26. The fixing roller 26 is preheated to a predetermined temperature by the heat source 27 before starting the printing operation. The temperature measured during printing is sensed by means of a temperature detector such as a thermistor (not shown for the sake of simplicity), and the fixing roller 26 is controlled based on the sensed temperature. That is, the fixing unit 13 is moved together with the process unit 12, and performs the fixing operation immediately after the image transfer by the process unit 12.
However, the conventional structure shown in FIGS. 1A and 1B has the following disadvantages.
As has been described previously, the toner image formed on the image carrier 21 is transferred to the recording paper 17 and immediately after this the transferred image is fixed thereon by the fixing roller 26, which is pressed against the recording paper 17. Hence, if the relative paper feeding quantity of the fixing roller 26 (which can be considered as if the fixing roller 26 is kept stationary) is not equal to that of the image carrier 21 (carriage 11), the recording paper 17 sandwiched between the transporting rollers 16a and 16b and fixed thereby may becomes a wavy state during the fixing process. Hence, it becomes possible to record images or characters on the correct positions of the recording paper 17.
In order to make the relative paper feeding quantity of the fixing roller 26 equal to that of the image carrier 21, a motor may be provided in the carriage 11. However, the use of the above motor prevents down-sizing of the carriage and increases the force of inertia due to an increase in the weight of the carriage 11. Hence, the use of the motor prevents the printing in the correct positions
The image carrier 21 is needed to return to the home position (print starting position) after the printing operation on one line is completed while the image carrier is rotated clockwise. If the image carrier 21 is returned to the home position in a state in which the image carrier 21 is in contact with the recording paper 17. Hence, the image carrier 21 may be damaged and the printing quality may be degraded. Further, the above contact functions as a load on the returning movement of the image carrier 21, and prevents the high-speed returning movement thereof.